Financial Instruments - Additional Information (Detail) (USD $)	3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012	Dec. 31, 2012
Schedule of Investments [Line Items]
Cash, cash equivalents and short-term investments	$ 380,500,000		$ 380,500,000		$ 442,600,000
Net unrealized gain (loss), net of tax, related to temporary decrease in value of available-for-sale securities			(100,000)
Impairment charges	0	0	0	0
Operating Accounts
Schedule of Investments [Line Items]
Cash, cash equivalents and short-term investments	$ 47,800,000		$ 47,800,000		$ 47,000,000